The degeneration of the intervertebral disk, such as the degeneration of the nucleus pulpous of the disk, results in a loss of height in the affected disk space which is associated with a weakening of the annulus fibrous and of the ligaments. As a consequence, the spinal column may become instable and more susceptible to horizontal displacement of the vertebral bodies with respect to one another. Such movement may result in impairments of the nerve roots in this region and/or of the spinal marrow with pain resulting therefrom.
The principle treatment of these symptoms consists of the surgical removal of the nucleus pulpous and the insertion of support bodies in order to restore the normal height of the disk space. While there are a number of traditional systems and methods for inserting support bodies, there are a variety of demands on both the surgeon performing an intervertebral disk procedure, the tools used to secure and insert the interbody spacers, and on the spinal spacers themselves.
In many cases, a surgeon inserts an interbody spacer within a spinal column using an insertion tool configured to threadably secure an interbody spacer. Many insertion tools include a male-threaded tip configured to mate with a corresponding female-threaded component formed in or on an interbody spacer. Alternatively, an insertion tool may be configured to grasp the perimeter of an interbody spacer during insertion.
Traditional interbody spacers, such as fusion cages, may comprise bodies manufactured from PEEK (polyehteretherketone). PEEK is commonly used because it does not distort MRI and CT images of the vertebrae. However, because new bone growth does not adhere well to PEEK, bone fusion with a PEEK cage typically relies on bridging bone growth through holes in the cage to provide stabilization.
The reliance on bridged bone growth through holes in the cage leads to interbody spacers with a large percentage of open space relative to the supporting PEEK structure. The more open space an interbody spacer has, the higher the load on each portion of the interbody spacer will be. In order to increase the strength of the interbody spacer, increase the stiffness of the interbody spacer, and/or to foster bone growth, various alternative materials have been developed for use in interbody spacers.
Interbody spacers made of PEEK can be readily threaded in order to accommodate insertion tools with male-threaded members. However, some materials that are suitable for interbody spacers, such as silicon nitride (Si3N4) and various other ceramics that are otherwise viable materials for use as spinal implants, are either not threadable, not easily threaded, or result in threads that break under a load.